Mass spectrometry



July 29, 1958 c. F. ROBINSON MASS SPECTROMETRY 2 Sheets-Sheet 1 Filed March 28, 1955 IN V EN TOR. CHARLES f. ROBINSON am M A TTORNEYS y 1958 c. F. ROBINSON MASS SPECTROMETRY Filed Mrch 2a, 1955 2 Sheets-Sheet 2 INVENTOR. CHARLES E ROBINSON 4 TTOR/V E KS United States Patent MASS SPECTROMETRY Charles F. Robinson, Pasadena, Calif., assignor to Consolidated Engineering Corporation, Pasadena, Calif., a corporation of California Application March 28, 1955, Serial No. 497,097

5 Claims. (Cl. 25041.9)

This invention relates to mass spectrometry and particularly to improvements in the structure of the ion source of a cycloidal or crossed field mass spectrometer.

The principle of operation of a cycloidal mass spectrometer is as follows: If a charged particle is introduced into a magnetic field it will move in a circular path to return to its point of origin. This is true regardless of the mass of the particle, with particles of increasing mass traveling in circles of increasing radius, but in each instance returning to the point of origin. If a uniform electric field is imposed across the space defined by the magnetic field and normal to the magnetic field, the ions pursue a path which may be considered as rigorously circular in a coordinate system moving with uniform velocity. The movement of the coordinate system is'a function of the ratio of the electric and magnetic field strengths. If ions of a par ticular mass are introduced into such a field system they will complete one turn of their circular motion in a time which depends directly on the mass of the particle and, if the electric field strength is uniform so that the coordinate systems corresponding to each particle move at the same velocity, the particles will converge to a series of rigorous point foci after any integral number of turns in the magnetic field and regardless of their velocity or direction of. travel at the moment of introduction into the field.

Since the time required for ions to complete one turn of their circular motion depends directly on mass, and since under the conditions of uniform field strength specified the rate of motion of the coordinate system is invariant to the mass of the particles involved, the focal point of the heavy particles will be displaced farther from the point of origin than the focal point of the lighter particles. This is the basic concept of the cycloidal mass spectrometer.

A mass spectrometer of this type has a principal characteristic advantage. Because ofthe fact that ions of -a given mass will converge to rigorous point foci after any integral number of turns regardless of their velocity or direction of travel at the moment of introduction into the crossed field, the cycloidal mass spectrometer is not subject to aberration and is insensitive not only to the energy spread of ions introduced into the field but also to the angular divergence of the ion beam at the point of introduction into the field.

I have now ascertained that because of this unique characteristic of the cycloidal mass spectrometer it is possible to construct an ion source which, by taking advantage of this characteristic, is far superior to ion sources suitable for use in other forms of mass spectrometers. The ion source of the invention differs from conventional ion sources in a radically difierent concept of spacing between the components of the source. The source comprises the usual elements including a repeller electrode, a first apertured accelerating electrode, means for developing an ionizing electron beam to traverse the recelerating electrode, the aperture of which represents an exit slit through which ions emerge from the ion system into a cycloidal analyzer. The term cycloidal analyzer is used herein as referring to the analyzing chamber of a cycloidal mass spectrometer for which the disclosed ion source is designed.

In this instance the ion source is so dimensioned that the maximum distance of ion travel from the most remote edge of the electron beam to the exit slit, i. e. the aperture in the second accelerating electrode, is .070 inch and the spacing between the first and second accelerating electrodes is such that the angular divergence of the ion beam at the exit slit is not less than 3 on each side of the centerline of the beam and preferably in the neighborhood of 6. This structure differs from the conventional ion source to a large extent since if, in other forms of mass spectrometers, the ion beam were allowed to have a divergent angle anywhere near that herein specified, resolution of the instrument would be substantially destroyed.

The ion source structure herein defined has three main advantages over the ion sources heretofore employed:

(1) Reduced discrimination efiects.ln any mass spectrometer ion source, conditions-are adjusted so that a particular ion mass will emerge therefrom under conditions which are optimum for that ion. The same set of conditions will not be optimum for another ion which differs appreciably in mass or in initial velocity from the first, so that it is inherent that any ion source will show discrimination between ions of different mass and between ions of the same'mass but of difierent initial velocities.

It follows that the longer the ions remain in the source the greater is the opportunity for the ion source to create these discrimination effects.

The limitation in sector type mass spectrometers has been that for a given slit size the angular divergence of the ion beam becomes larger as the slit spacing is reduced and the inability of a sector type instrument to resolve widely divergent ion beams has precluded slit spac-' it possible to employ a dual collector system in a mass spectrometer of this type, which system is difficult or impossible to achieve it conventional slit spacings are employed.

(2) Reduced space charge efiects.In a conventional ion source there is a considerable degree of mass separation between the first and second slits. When the instrument is set to pass ions of one mass, ions of widely differing mass tend to strike and discharge upon the terminal accelerating electrode and thereby never emerge from the ion source. The space charge of these ions can have the eifect of a very large gas interference in the region between the first and second slits. This efiect varies approximately as the square of the slit spacing and has been reduced by a factor of 16 in the cycloidal mass spectrometer by reason of the construction of the ion source as herein defined.

(3) Increased sensitivity.-The sensitivity of an ion source tends to vary with the solid angle subtended at the first slit by thesecond slit, although there are many increases the sensitivity by an approximately similar factor.

The invention will be clearly understood from the following detailed description taken in conjunction with the accompanying drawing, in which:

Fig. l is a longitudinal sectional elevation through a cycloidal focusing mass spectrometer;

Fig. 2 is a schematic diagram of the electrical circuitry of the spectrometer of Fig. 1; and

Fig. 3 is a greatly enlarged schematic view of the ion source in accordance with the invention showing the critical conditions above defined.

The cycloidal mass spectrometer shown in Fig. 1 comprises an evacuable envelope 5 provided with conduit means 6 for connection to an evacuating system (not shown) and sample inlet means 7. A plurality of electrodes 8, 9, 10, 11, 12, 13 and Marc supported in the envelope from a framework 15 by series of pins 16, 17, 17A, 18, 19, 20. The several electrodes are spaced and insulated from each other by insulating spheres 21, 22, a

23, etc. The electrode structure defines a chamber 24 having an inlet slit 25 and a resolving slit 26 spaced from each other on a common or so-called focal plane. An ion source 28 is supported adjacent the chamber 24 and includes a terminal accelerating electrode 29 defining the above mentioned inlet slit 25.

As' shown schematically in Fig. 2, the ion source includes a chamber 30, electron gun 31, an electron target 32, a repeller electrode 33, and afirst accelerating electrode 34 having an aperture 35, and the electrode 29 defining the inlet aperture 25 to the cycloidal analyzer. The above recited elements of the ion source are arranged so that molecules in the chamber 30 are ionized by electron beam 36 traveling between the gun 31 and target 32 and, under the influence of the potential between the repeller electrode .33 and the accelerating electrodes 34 and 29, the ions are propelled through aperture 25 into the cycloidal analyzer.

Electron gun 31 and target 32 are conventionally interconnected through an emission regulator circuit 38 so that the ionizing electron beam 36 is maintained at substantially uniform density. Many emission regulator circuits are known in the art as conventional adjuncts to a great number of commercial mass spectrometers.

Appropriate potentials are impressed on the several electrodes 8, 9, 10, etc. by means of a voltage divider network also shown schematically in Fig. 2. A D. C. power supply 40 is connected across a capacitor 41. A voltage divider 42 is connected in parallel across the capacitor 41 'and the several electrodes 8, 9, 10, 11, 12, 13

and 14 are connected to the divider network 42 as illustrated. A mass spectrum can be scanned by char ing the capacitor 41 and allowing the charge to decay across the voltage divider 42. The several field forming electrodes will remain at the same relative potentials but the field strengths will diminish as the capacitor 41 discharges and different ion trajectories will be brought to focus at the resolving aperture 26.

Electrode 12 is provided with a cavity 44 into which the resolving aperture 26 opens and in which a collector electrode 45 is mounted. Ions focusing on the resolving aperture 26 will collect on and discharge at the collector electrode 45 and will be sensed by a sensing device 46 which may be conventional. Suitable electrical leads are brought through a wall of the envelope 26 in conventional manner for connection of the circuit of Fig. 2 to the various portions of the ion source and the field forming electrodes and connection of the sensing device to the collector electrode. An electrical conduit 47 is shown as accomplishing this purpose, the individual electrical'leads brought into the conduit not being distributed in the drawing for purposes of clarity.

The entire instrument is immersed in a magnetic field developed between magnet pole 48 and a companion pole piecetnot shown) disposed on the opposite side of the ture 25 approximately .060 inch.

envelope 5. The two magnet poles develop a magnetic fieldnormal to the electrical field existing between the several electrodes 8, 9, 10, etc.

The particular construction of the ion source in accordance with the present invention is shown in the enlarged schematic view of Fig. 3, which shows the repeller electrode 33, electron beam 36 incross-section, first accelerating electrode 34 provided with aperture 35 and the second accelerating electrode 29 provided with the aperture 25, this being the inlet slit into the cycloidal analyzer. The limiting conditions of the construction of the ion source are illustrated on the drawing showing that the maximum travel of ions from the trailing surface of the electron beam 36 to the aperture 25 in the terminal accelerating'electrode 29 is 0.70 inch. This is a maximum value and preferably this distance is not in excess of .060 inch.

At the same time the spacing between electrodes 29 and 34 and the dimensions of the respective apertures 25 and 35 is such that the angular divergence of the ion beam, the beam being shown by dotted lines extending from the electron beam 36 through the aperture 25, is not less than 3 from the centerline. Again this is a limiting figure and preferably this relationship is such that the angular divergence is approximately 6 from the centerline.

In a typical instrument embodying the preferred structural limitations of the invention the aperture 35 may be approximately .007 of an inch in width, the aperture 25 approximately .003 of an inch, the spacing between the two apertures about .035 inch and the distance between the trailing surface of the electron beam 36 and the aper- It will be apparent that this construction differs radically from that of conventional ion sources and as previously explained I have discovered that the construction herein set forth greatly improves the operation of a cycloidal mass spectrometer and is at the same time possible because of the unique focusing characteristics of this type of instrument.

In the operation of the illustrated device, sample molecules introduced to the ion source are ionized by the electron beam and under the influence of a propelling potential established across the ion source between the repeller electrode and the first and second accelerating electrodes are expelled from the source into the cycloidal analyzer. Responsive to the transversely oriented magnetic and electrical fields impressed across the analyzer, the ions pursue cycloidal trajectories in the chamber. The pitch of the ion trajectories is a function both of mass and field strength,'the latter being controlled to focus ions of a given predetermined mass at the resolving slit. The in-focus ions are collected at the collector electrode and the resultant discharge current is sensed in any conventional fashion.

To scan a mass spectrum, either of the transverse magnetic and electrical fields may be varied to successively focus ions of a different mass on the resolving slit. The ion source meeting the structural limitations herein specified greatly improves the operation of an instrument of this type in avoiding ion discrimination normally encountered in the source, reducing the sensitivity of the instrument to space charges in the ion source and increasing the absolute sensitivity of the instrument by passing a greater number of the total ions formed in the source into the cycloidal analyzer.

One form of cycloidal mass spectrometer has been illustrated and described in detail. However, it is understood that the invention is not directed to the specific form of instrument as presented apart from the specific construction of the ion source as recited. The particular ion source is applicable to any of the many forms of cycloidal mass spectrometers, the source being limited only to use with a double focusing instrument of this particular generic type.

I claim:

1. In combination with a cycloidal analyzer, an ion source comprising means defining an ionizing region, means for developing an ionizing electron beam across the ionizing region, a repeller electrode supported on one side of the electron beam, first and second apertured electrodes spaced from each other on the opposite side of the electron beam, means for admitting molecules to be ionized, and means for developing a potential between the electrodes to cause ions formed by the electron beam to flow successively through the apertures in the first and second accelerating electrodes, the spacing of the means forming the ion beam and the second electrode being such that the distance the ions travel from the point of formation to the aperture in the second accelerating electrode does not exceed .070 inch.

2. In combination with a cycloidal analyzer, an ion source comprising means defining an ionizing region, means for developing an ionizing electron beam across the ionizing region, a repeller electrode supported on one side of the electron beam, first and second apertured electrodes spaced from each other on the opposite side of the electron beam, means for admitting molecules to be ionized, and means for developing a potential between the electrodes to cause ions formed by the electron beam to flow successively through the apertures in the first and second accelerating electrodes, the aperture in the second electrode being smaller than the aperture in the first electrode and the electrodes being closely spaced so that the angle subtended by an ion beam at the aperture in the second electrode is at least 3 on each side of center.

3. In combination with a cycloidal analyzer, an ion source comprising means defining an ionizing region, means for developing an ionizing electron beam across the ionizing region, a repeller electrode supported on one side of the electron beam, first and second apertured electrodes spaced from each other on the opposite side of the electron beam, means for admitting molecules to be ionized, and means for developing a potential between the electrodes to cause ions formed by the electron beam to flow successively through the apertures in the first and second accelerating electrodes, the spacing of the means forming the ion beam and the first and second electrodes and the sizes of the electrode apertures being such that the distance the ions travel from the point of formation to the aperture in the second accelerating electrode does not exceed .070 inch, and the angle subtended by an ion beam at the aperture in the second electrode is at least 3 on each side of center.

4. In combination with a cycloidal analyzer, an ion source comprising means defining an ionizing region, means for developing an ionizing electron beam across the ionizing region, a repeller electrode supported on one side of the electron beam, first and second apertured electrodes spaced from each other on the opposite side of the electron beam, means for admitting molecules to be ionized, and means for developing a potential between the electrodes to cause ions formed by the electron beam to flow successively through the apertures in the first and second accelerating electrodes, the spacing of the first and second electrodes and size of the electrode apertures being such that the angular divergence of the ion beam at the aperture in the second electrode is about 6' on each side of center.

5. In combination with a cycloidal analyzer, an ion source comprising means defining an ionizing region, means for developing an ionizing electron beam across the ionizing region, a repeller electrode supported on one side of the electron beam, first and second apertured electrodes spaced from each other on the opposite side of the electron beam, means for admitting molecules to be ionized, and means for developing a potential between the electrodes to cause ions formed by the electron beam to flow successively through the apertures in the first and second accelerating electrodes, the apertures in the electrodes being about-.035 inch apart and the spacing of the means forming the ion beam and the second electrode being such that the distance the ions travel from the point of formation to the aperture in the second accelerating electrode is about .060 inch.

References Cited in the file of this patent UNITED STATES PATENTS 2,221,467 Bleakney Nov. 12, 1940 

